CN113791470B - Resonance grating waveguide structure and near-to-eye display device - Google Patents
Resonance grating waveguide structure and near-to-eye display device Download PDFInfo
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
- G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
- G02B6/0013—Means for improving the coupling-in of light from the light source into the light guide
- G02B6/0015—Means for improving the coupling-in of light from the light source into the light guide provided on the surface of the light guide or in the bulk of it
- G02B6/0016—Grooves, prisms, gratings, scattering particles or rough surfaces
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- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
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- G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
- G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
- G02B6/0033—Means for improving the coupling-out of light from the light guide
- G02B6/005—Means for improving the coupling-out of light from the light guide provided by one optical element, or plurality thereof, placed on the light output side of the light guide
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- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/01—Head-up displays
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- G02B2027/0178—Eyeglass type
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Abstract
The invention discloses a resonant grating waveguide structure, comprising: at least one waveguide structure layer, each waveguide structure layer is provided with at least one pair of coupled-in resonance waveguide grating and coupled-out resonance waveguide grating; wherein, coupling-in resonance waveguide grating and coupling-out resonance waveguide grating are multilayer structure, all by from last to stacking grating layer, low refracting index layer, high refracting index layer and the transparent waveguide basement that sets up in proper order down and constitute: the grating period of the grating layer is in the sub-wavelength range; the transparent waveguide substrate of the coupling-in resonance waveguide grating and the coupling-out resonance waveguide grating is the same transparent waveguide substrate which is shared. The problem of uneven color caused by the dispersion effect can be avoided, full-color display with high color purity and uniform color is realized, and the maximum field angle is realized.
Description
Technical Field
The invention relates to the field of near-eye display, in particular to a resonance grating waveguide structure and a near-eye display device which are applied to near-eye display devices such as AR and VR and can realize high-quality full-color display.
Background
Augmented Reality (AR) and Virtual Reality (VR) are hot technological fields that have attracted much attention in recent years, and with the development of 5G technology, the future market prospect thereof has become increasingly clear, so that near-to-eye display technology has been rapidly developed. Augmented Reality (AR) is a technology that overlays digitized information (including text, images, video, etc.) onto the real physical world. The AR brings the integration of a digital world and a physical world, and has the remarkable characteristics of strong perspective and strong mobility, the AR glasses imaging system cannot block the front of a sight line, normal observation on a real environment cannot be influenced, and practical feelings of moving scenes are brought to a user. The display system adopted by AR glasses in the market at present is a combination of various micro display screens and optical elements such as prisms, Bird Bath, free curved surfaces, optical waveguides and the like, and can couple virtual images into human eyes. The optical waveguide technology is considered as a necessary optical scheme for the AR glasses to move to the consumer level due to the characteristics of lightness, thinness and high penetration to external light, and has great development potential in terms of optical effect, appearance beautification and mass production prospect.
Optical waveguides can be generally classified into geometric optical waveguides and diffractive optical waveguides. Due to the flexibility of the grating in design and production, diffractive optical waveguide technology has advantages over geometric optical waveguides in terms of mass productivity and product yield. Thanks to the development of micromachining and "planar optics" technologies, surface relief grating waveguides are currently adopted and proven to be mass-producible by a number of large international companies. The advantages of the grating waveguide are mainly embodied in that the area, the shape and the arrangement mode of the grating region can be flexibly adjusted according to the optical parameter requirements and the appearance design of the AR glasses. Because the total reflection principle the same as that of the optical fiber technology is adopted, the grating waveguide display device can be as light, thin and transparent as common glasses lenses.
The present grating waveguide has dispersion problems due to the selectivity of the diffractive element itself for angle and wavelength, which is mainly manifested by field angle (FOV) and color non-uniformity within the moving eye box, i.e., "rainbow effect". Fig. 1 is a schematic diagram of a single-layer grating waveguide, in which, since the same diffraction grating has different diffraction angles corresponding to different wavelengths of light, red, green, and blue (RGB) lights in different bands are diffracted by an incident grating, the corresponding diffraction angles θ will be different, and θ existsR＞θG＞θBThe light of each color passes through different optical paths in the waveguide after completing total reflection once, and the total reflection times are different in the whole transmission process, so that the RGB color proportion seen when the eyes move to different positions of the eye moving frame is not uniform when the light passes through the emergent grating; in addition, even the diffraction efficiency of light of the same color varies with the incident angle, which causes the distribution ratio of red, green, and blue lights to be different over the entire field angle (FOV), and thus a "rainbow effect" occurs. To improve dispersion problems and achieve maximum FOV for single layer grating waveguides, solutions are currently usedThe solution is that the grating waveguide with multilayer structure as shown in fig. 2 couples red, green and blue lights into three layers of waveguides respectively, each layer of waveguide only conducts light with single color, the corresponding in-out grating is optimized for a certain color, and the grating parameters are adjusted to make the diffraction angle reach the maximum FOV, so that the color uniformity in the range of the final eye-moving frame can be improved, and the rainbow effect is reduced. However, since the grating optimized for a certain color of light still diffracts light of other colors within a certain angle range, and light of each color does not correspond to a wavelength but covers a certain wavelength range, the dispersion problem can only be improved to a certain extent, and the "rainbow effect" is also improved to a certain extent but cannot be eliminated, and meanwhile, the multilayer-structure grating waveguide scheme also has the problem that the wavelength band and the incidence angle range (i.e. the maximum field angle, FOV) of the covered color of light are difficult to be compatible.
Therefore, how to overcome the dispersion problem caused by the diffraction element, and to realize high color purity, uniform color full-color display and maximum FOV is a problem to be solved urgently at present.
In view of the above, the present invention is particularly proposed.
Disclosure of Invention
The invention aims to provide a resonance grating waveguide component and a near-to-eye display device capable of realizing full-color display, which can avoid the problem of non-uniform color caused by dispersion effect, realize high-color purity and uniform-color full-color display and realize the maximum field angle, and solve the technical problems in the prior art.
The purpose of the invention is realized by the following technical scheme:
an embodiment of the present invention provides a resonant grating waveguide structure, including:
at least one waveguide structure layer, each waveguide structure layer is provided with at least one pair of coupled-in resonance waveguide grating and coupled-out resonance waveguide grating; wherein,
the coupling-in resonant waveguide grating and the coupling-out resonant waveguide grating are of multilayer structures and are formed by sequentially stacking a grating layer, a low refractive index layer, a high refractive index layer and a transparent waveguide substrate from top to bottom: the grating period of the grating layer is in a sub-wavelength range lower than the wavelength of incident light in vacuum;
the transparent waveguide substrate 110 of the in-coupling resonant waveguide grating and the out-coupling resonant waveguide grating is the same transparent waveguide substrate.
The embodiment of the invention also provides a near-to-eye display device, and the resonance grating waveguide structure is adopted as a grating waveguide device.
Compared with the prior art, the resonance grating waveguide structure and the near-to-eye display device provided by the invention have the beneficial effects that:
at least one pair of coupling-in resonant waveguide grating and coupling-out resonant waveguide grating is arranged on each waveguide structure layer, and transparent waveguide substrates of the coupling-in resonant waveguide grating and the coupling-out resonant waveguide grating are the same transparent waveguide substrate which is shared, so that the coupling-in resonant waveguide grating and the coupling-out resonant waveguide grating are separately arranged on the surface of the same transparent waveguide substrate at intervals, resonance diffraction of coupling-in and coupling-out of incident light is realized, resonance diffraction can be generated on light with corresponding wavelength and incident angle, and the diffraction efficiency of the light with other wavelengths and incident angles is close to zero; moreover, as the grating period of the coupling-in resonance waveguide grating and the coupling-out resonance waveguide grating is in a sub-wavelength range (namely, the grating period is lower than the wavelength of incident light in vacuum), the diffraction order which can be conducted in the transparent waveguide substrate is limited, which is beneficial to reducing stray light and leading the waveguide structure to have higher penetrability to ambient light; by adjusting the grating parameters of the coupled-in resonant waveguide grating and the coupled-out resonant waveguide grating, the red, green and blue lights can be subjected to resonant diffraction under the same diffraction angle, so that dispersion-free resonant diffraction is realized, the dispersion effect problem of diffraction elements such as the conventional grating waveguide is further solved, the rainbow effect is fundamentally eliminated, and high-color-purity full-color display is realized.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
FIG. 1 is a schematic view of a prior art optical waveguide providing a single-layer structure;
FIG. 2 is a schematic diagram of a prior art technique for providing a multilayer structured optical waveguide;
FIG. 3(a) is a schematic diagram of a single layer resonant grating waveguide structure provided by an embodiment of the present invention;
FIG. 3(b) is a schematic diagram of a three-layer resonant grating waveguide structure provided by an embodiment of the present invention;
fig. 4 is a schematic structural diagram of a coupling-in and coupling-out resonant waveguide grating of a resonant grating waveguide structure according to an embodiment of the present invention;
fig. 5 shows the RCWA simulation result of the variation of diffraction efficiency of each stage of the incoupling resonant waveguide grating with the incident angle (incident light wavelength 520nm, TE polarization) of the resonant grating waveguide structure according to the embodiment of the present invention;
FIG. 6 shows the RCWA simulation results of the variation of diffraction efficiency of each stage of the coupled-in resonant waveguide grating with the incident angle (incident light wavelength 520nm, TM polarization) of the resonant grating waveguide structure according to the embodiment of the present invention;
fig. 7 is a result of RCWA simulation calculation in which the coupled-in resonant waveguide grating of the resonant grating waveguide structure according to the embodiment of the present invention performs resonant diffraction when the red, green, and blue light is incident: under specific grating parameters, the red, green and blue lights are subjected to resonance diffraction under the same incident angle and the diffraction angles are the same, namely, dispersion-free resonance diffraction. (red wavelength 630nm, green 520nm, blue 450nm, TE polarization);
fig. 8 is an RCWA simulation result of-1 order resonant diffraction efficiency of an incoupling resonant waveguide grating as a function of wavelength for a resonant grating waveguide structure designed for red, green, and blue light according to an embodiment of the present invention;
fig. 9 shows the results of RCWA simulation of-1 order resonance diffraction for a resonant grating waveguide structure according to an embodiment of the present invention: adjusting the grating parameters to ensure that the resonance diffraction angle of the grating is continuously changed within a certain range (the wavelength of incident light is 520 nanometers, and TE polarization);
FIG. 10 provides RCWA simulation results of wavelength dispersion characteristics of a resonant grating waveguide structure (grating design for green light incidence at a wavelength of 520 nm) according to an embodiment of the present invention;
fig. 11 is an RCWA simulation result of the resonant grating waveguide structure according to the embodiment of the present invention, where the resonant diffraction angle varies with the thickness of LiF of the low refractive index layer and the grating period: the thickness of the low refractive index layer and the grating period are adjusted to enable the resonance diffraction angle to be continuously changed within a certain range (the wavelength of incident light is 520 nanometers, TE polarization);
FIG. 12 shows the variation of the resonance diffraction angle of the resonant grating waveguide structure with the thickness of the high refractive index layer TiO2 (incident light wavelength 520nm, TE polarization);
in fig. 2 to 4: 100-a first waveguide structure layer; 120-incoupling of a resonant waveguide grating; 110-a waveguide substrate; 22-high refractive index layer; 24-a low refractive index layer; 26-a grating layer; 130-out-coupling resonant waveguide grating; 200-a second waveguide structure layer; 300-third waveguide structure layer.
Detailed Description
The technical scheme in the embodiment of the invention is clearly and completely described below by combining the attached drawings in the embodiment of the invention; it is to be understood that the described embodiments are merely exemplary of the invention, and are not intended to limit the invention to the particular forms disclosed. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.
The terms that may be used herein are first described as follows:
the term "and/or" means that either or both can be achieved, for example, X and/or Y means that both cases include "X" or "Y" as well as three cases including "X and Y".
The terms "comprising," "including," "containing," "having," or other similar terms of meaning should be construed as non-exclusive inclusions. For example: including a feature (e.g., material, component, ingredient, carrier, formulation, material, dimension, part, component, mechanism, device, process, procedure, method, reaction condition, processing condition, parameter, algorithm, signal, data, product, or article of manufacture), is to be construed as including not only the particular feature explicitly listed but also other features not explicitly listed as such which are known in the art.
The term "consisting of … …" is meant to exclude any technical feature elements not explicitly listed. If used in a claim, the term shall render the claim closed except for the inclusion of the technical features that are expressly listed except for the conventional impurities associated therewith. If the term occurs in only one clause of the claims, it is defined only to the elements explicitly recited in that clause, and elements recited in other clauses are not excluded from the overall claims.
Unless expressly stated or limited otherwise, the terms "mounted," "connected," and "secured," etc., are to be construed broadly, as for example: can be fixedly connected, can also be detachably connected or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms herein can be understood by those of ordinary skill in the art as appropriate.
The terms "central," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," and the like are used in an orientation or positional relationship that is indicated based on the orientation or positional relationship shown in the drawings for ease of description and simplicity of description only, and are not intended to imply or imply that the referenced device or element must have a particular orientation, be constructed and operated in a particular orientation, and are therefore not to be considered limiting herein.
The resonant grating waveguide structure provided by the present invention is described in detail below. Details which are not described in detail in the embodiments of the invention belong to the prior art which is known to the person skilled in the art. Those not specifically mentioned in the examples of the present invention were carried out according to the conventional conditions in the art or conditions suggested by the manufacturer. The reagents or instruments used in the examples of the present invention are not specified by manufacturers, and are all conventional products available by commercial purchase.
As shown in fig. 3 and 4, an embodiment of the present invention provides a resonance grating waveguide structure, which can solve the problem of color non-uniformity caused by a dispersion effect in a grating waveguide display device, and implement full-color display of a grating waveguide with high color purity and uniform color and implement a maximum field angle, and the resonance grating waveguide structure includes:
at least one waveguide structure layer, each waveguide structure layer is provided with at least one pair of coupled-in resonant waveguide grating 120 and coupled-out resonant waveguide grating 130; wherein,
the incoupling resonant waveguide grating 120 and the outcoupling resonant waveguide grating 130 are both multilayer structures, and are formed by a grating layer, a low refractive index layer, a high refractive index layer and a transparent waveguide substrate 110 which are sequentially stacked from top to bottom: the grating period of the grating layer is in a sub-wavelength range lower than the wavelength of incident light in vacuum;
the transparent waveguide substrate 110 of the in-coupling resonant waveguide grating 120 and the out-coupling resonant waveguide grating 130 is a common transparent waveguide substrate.
If one waveguide structure layer is provided, the grating waveguide structure is a monochromatic display grating waveguide structure (see the first waveguide structure layer in fig. 3 (a)).
Referring to fig. 3(b), further, the waveguide structure layer has three layers, the three waveguide structure layers (i.e., the first waveguide structure layer 100, the second waveguide structure layer 200, and the third waveguide structure layer 300 in fig. 3 (b)) are stacked from top to bottom, and each waveguide structure layer diffracts and propagates light of a different color correspondingly. If three waveguide structure layers respectively correspond to light of red, green and blue in diffraction propagation, the waveguide structure is a grating waveguide structure for full-color display. It is understood that to expand the FOV, more pairs of in-coupling resonant waveguide grating 120 and out-coupling resonant waveguide grating 130 may be disposed in each waveguide structure layer, so that the resonant diffraction angle of the grating continuously changes within a certain range.
It will be appreciated that more layers of waveguide structure may be provided to correspond to different colours of light.
In the waveguide structure, the coupling-in resonant waveguide grating 120 and the coupling-out resonant waveguide grating 130,
the refractive index of the grating layer is smaller than that of the high refractive index layer and larger than that of the low refractive index layer;
the transparent waveguide substrate 110 has a refractive index smaller than that of the high refractive index layer and larger than that of the low refractive index layer.
In the waveguide structure, the refractive index of the grating layer is the same as or different from the refractive index of the transparent waveguide substrate 110.
In the waveguide structure, the refractive index of the high refractive index layer is 2.0-4.0;
the refractive index of the low refractive index layer is 1.0-1.5;
the refractive index of the grating layer is 1.5-2.0;
the refractive index of the transparent waveguide substrate 110 is 1.5-2.0.
It is understood that the refractive index of each layer is only a preferable refractive index, and other materials satisfying the refractive index relationship between the layers may realize each layer of the incoupling and outcoupling resonant waveguide gratings 120 and 130.
In the waveguide structure, the height of the grating layer is 40-200 nanometers;
the thickness of the low refractive index layer is 30-200 nanometers;
the thickness of the high-refractive-index layer is 30-300 nanometers.
In the waveguide structure, in the incoupling resonant waveguide grating 120 and the outcoupling resonant waveguide grating 130, the grating period, height, duty ratio of the grating layer and the thickness of the high-refractive index layer and the low-refractive index layer have the parameter values that all satisfy the requirement that the resonant waveguide grating generates a resonant mode for light with corresponding wavelength and incident angle;
the thicknesses of the high refractive index layer and the low refractive index layer meet phase matching, and energy of a resonance mode can be coupled into a high diffraction order and the highest diffraction efficiency can be achieved.
In the waveguide structure, the ratio of the incident light wavelength λ of the coupled-in resonant waveguide grating 120 and the coupled-out resonant waveguide grating 130 of each waveguide structure layer arranged for different colors of light to the grating period a of the grating layer satisfies λR÷aR＝λG÷aG＝λB÷aBThat is, the diffraction angles are the same at the same incident angle; wherein λ isRIncident light wavelength of incident red light, aRA grating period set for red light; lambda [ alpha ]GIncident wavelength of incident green light, aGA grating period set for green light; lambda [ alpha ]BIncident light wavelength of incident blue light, aBThe grating period set for blue light. On the basis, the structural parameters of the resonant waveguide grating are adjusted, so that the red light, the green light and the blue light are subjected to diffraction resonance under the same incident angle, and the diffraction angle in the transparent waveguide is also the same at the moment according to a grating formula, so that the rainbow effect caused by dispersion is fundamentally eliminated, and the full-color display of the grating waveguide with high color purity and uniform color is realized.
In the waveguide structure, the grating layer may be exposed to air (vacuum) or covered by a low-refractive-index light-transmitting outer layer.
In summary, in the resonant grating waveguide structure of the embodiment of the present invention, the coupled-in resonant waveguide grating and the coupled-out resonant waveguide grating of the multilayer structure sharing the same transparent waveguide substrate are arranged, so that resonant diffraction is achieved for incident light at corresponding wavelengths and incident angles, and the problem of uneven grating waveguide color caused by a dispersion effect is solved, so that non-dispersion color display is achieved; the light efficiency is improved, background stray light is reduced, and the light transmittance of the grating waveguide display device is improved.
The embodiment of the invention also provides a near-to-eye display device, which adopts the resonance grating waveguide structure as a grating waveguide device. The resonance grating waveguide structure of the non-dispersion color display is adopted, so that the display effect of the near-eye display device is improved.
In order to more clearly show the technical solutions and the technical effects provided by the present invention, the following describes the resonant grating waveguide structure provided by the present invention in detail with specific embodiments.
Examples
The embodiment of the invention provides a resonance grating waveguide structure which is suitable for near-eye display devices such as AR and VR and the like and realizes the dispersion-free RGB full-color display of grating waveguides. As shown in fig. 3(b), the resonant grating waveguide structure combines the sub-wavelength grating structure and the transparent optical waveguide, and specifically includes: three waveguide structure layers 100, 200, 300, which are stacked together from top to bottom, are respectively corresponding to light of different colors of diffraction transmission red, green, blue, and each waveguide structure layer has the same composition, each waveguide structure layer is provided with at least one pair of coupling-in resonant waveguide grating 120 and coupling-out resonant waveguide grating 130, and a transparent waveguide substrate 110 shared by the coupling-in resonant waveguide grating and the coupling-out resonant waveguide grating, namely the resonant waveguide grating and the transparent waveguide substrate are integrated into an integral structure, and the transparent waveguide substrate 110 is a shared substrate for the coupling-in resonant waveguide grating and the coupling-out resonant waveguide grating; in each waveguide structure layer, the grating parameters of the incoupling resonant waveguide grating 120 and the outcoupling resonant waveguide grating 130 are the same, and the grating areas may be different, and are respectively used for light incoupling and outcoupling.
In the above-mentioned resonant grating waveguide structure, in each waveguide structure layer, the coupled-in resonant waveguide grating 120 and the coupled-out resonant waveguide grating 130 are both multilayer structures, and the structures of the two structures are the same, and the description will be given by taking an example of the structure of the coupled-in resonant waveguide grating 120, which is composed of a grating layer 26, a low refractive index layer 24, a high refractive index layer 22 and a common waveguide substrate 110 stacked from top to bottom; the grating period of the grating layer 26 is in the sub-wavelength range, which is lower than the wavelength of incident light in vacuum, and the grating layer with the sub-wavelength period enables the diffraction order which can be conducted in the transparent waveguide substrate to be limited, thereby being beneficial to reducing stray light; the grating period, the height and the duty ratio of the grating layer and the thicknesses of the high refractive index layer and the low refractive index layer meet the requirement that the resonant waveguide grating generates a resonant mode on light with corresponding wavelength and incident angle; the thicknesses of the high refractive index layer and the low refractive index layer meet phase matching, and the resonance mode energy can be coupled into a high diffraction order and the highest diffraction efficiency can be achieved.
In the coupling-in resonant waveguide grating 120 and the coupling-out resonant waveguide grating 130, the refractive index of the grating layer may be the same as or different from that of the transparent waveguide substrate, and the grating layer may be exposed to air (or vacuum) or coated with a low refractive index transparent outer layer.
Preferably, in the incoupling resonant waveguide grating 120 and the outcoupling resonant waveguide grating 130, the refractive index of the transparent waveguide substrate 110 is 1.5 to 2.0; the refractive index of the high refractive index layer is 2.0-4.0; the refractive index of the low refractive index layer is 1.0-1.5, and the refractive index of the grating layer is 1.5-2.0. This is a preferable refractive index of each layer, but the refractive index of each layer is not limited to these ranges, as long as the resonant waveguide grating can generate a resonant mode with respect to light of a corresponding wavelength and an incident angle.
In the coupled-in resonant waveguide grating 120 and the coupled-out resonant waveguide grating 130, the height of the grating layer is 40 to 200nm, and the thicknesses of the high refractive index layer and the low refractive index layer are both 30 to 300 nm.
The resonant grating waveguide structure of the invention has a transmission mode as a working mode, and the core is that the resonant grating waveguide structure is used as a resonant optical element, only generates resonant diffraction to light with corresponding wavelength and incident angle, and the diffraction efficiency to light with other wavelength and incident angle is close to zero. In the incoupling resonant waveguide grating 120 and the outcoupling resonant waveguide grating 130, the grating period, height, duty ratio of the grating layer and the thickness of the high and low refractive index layers satisfy the requirement that the resonant grating waveguide can generate a resonant mode, the values of the grating period, height, duty ratio of the grating layer and the thickness of the high and low refractive index layers can be given by RCWA calculation, the thicknesses of the high and low refractive index layers satisfy phase matching, the resonant mode energy is coupled into a high diffraction order, such as +1 or-1 order, and high-order resonant diffraction is realized. (for example, as shown in fig. 5, when the wavelength of incident light is 520nm, TE polarization, the grating period of the grating layer is 400nm, the height is 100nm, the duty ratio is 50%, the thickness of the low refractive index layer is 70nm, and the thickness of the high refractive index layer is 50nm, the-1-order resonance diffraction is obtained when the incident angle is 11 °, the diffraction efficiency is 60%, the full width at half maximum of the resonance spectrum is about 8.5 °, structural parameters corresponding to different incident wavelengths and resonance diffraction angles are different, and the parameters can be adjusted according to actual needs), and the resonance grating waveguide structure can enable red, green, and blue light to be subjected to resonance diffraction at the same diffraction angle by adjusting various parameters of the resonance waveguide grating structure, such as the grating period of the grating layer, so as to eliminate color nonuniformity caused by a dispersion effect, and realize the non-dispersive RGB full-color display of the grating waveguide.
When the wavelength lambda and the incident angle theta0When the incident light is incident on the incoupling resonant waveguide grating 120 with matched structural parameters, 1-order (or-1-order) resonant diffraction is generated on the incident light, the diffraction efficiency can be higher than 60%, and the diffraction efficiency on the incident light with other wavelengths and incident angles is lower than 10%, and the grating period, height, duty ratio and thicknesses of the high and low refractive index layers of the grating layer of the incoupling resonant waveguide grating 120 are determined according to the wavelength and incident angle theta of the incident light0Given by the RCWA calculation; the high selectivity of the resonance diffraction to wavelength and incident angle allows the resonant grating waveguide structure, the incoupling resonant waveguide grating 120, to have high penetration to visible light, and to be applied in AR display without affecting the observation of the real environment. As shown in fig. 5, taking the green light with the wavelength of 520nm and TE polarization as an example, when the green light with this wavelength is incident on the incoupling resonant waveguide grating 120, the-1 st order resonance diffraction is obtained at the incident angle of 11 ° (the-11 ° case +1 st order resonance diffraction), the diffraction efficiency is 60%, and the full width at half maximum of the resonance line is about 8.5 °; FIG. 6 is a diagram showing the diffraction efficiencies of 0-order and-1-order obtained when TM-polarized light with a wavelength of 520nm is incident on an incoupling resonant waveguide grating of the same structure. It can be seen that for the same grating structure, the incident light is TM polarized and no resonance diffraction is obtained as shown in fig. 5, where the 0-order diffraction efficiency varies little with angle, and the-1-order diffraction efficiency is below 10%. This shows that the incoupling resonant waveguide grating as shown in fig. 4 also has a high selectivity for the polarization state of the incident light, while being applicable as a polarizing beam splitter.
As can be seen from the above comparison, the resonant waveguide grating of the present invention (i.e., the in-coupling resonant waveguide grating 120 and the out-coupling resonant waveguide grating 130) resonates only when the incident light wave vector matches the eigenmode of the grating, i.e., resonant diffraction occurs at the corresponding incident wavelength and incident angle, while the light diffraction efficiency at other wavelengths and incident angles is negligible. The intrinsic mode of the resonant waveguide grating is determined by the structural parameters and the composition materials of the grating, when the wave vector of incident light diffracted by the grating is matched with the intrinsic mode of the grating, a waveguide mode is generated in the grating, the mode is generally bound in the grating structure, surface waves propagating along the surface of the grating are generated, and evanescent waves are generated in the environment medium around the grating. Due to the periodicity of the grating, the surface evanescent wave couples the energy of the waveguide mode generated in the grating into the guided mode of the grating through interaction with the grating wave vector, thereby generating resonance diffraction. The resonance waveguide grating structure can couple resonance energy into a high-order diffraction mode to realize high-order resonance diffraction, such as +1 (or-1) order resonance diffraction, and can meet the requirement of AR display on total reflection, which is the difference between the resonance waveguide grating structure and the conventional zero-order grating and is the key point of the invention. As described above, incident light is diffracted by the grating to excite a resonant waveguide mode in the resonant waveguide grating. The diffraction order of the excited waveguide mode and the resulting waveguide mode are typically evanescent waves in the ambient environment on both sides of the grating, so that the waveguide mode is diffracted again by the grating when coupled into the propagating diffraction order of the grating. Since the diffraction order exciting the resonant waveguide mode is generally 1 order, the efficiency of coupling the resonant waveguide mode into the zero order diffraction is generally the highest, while the efficiency of coupling into a high diffraction order such as-1 order is much lower, so the resonant waveguide grating generally operates in the zero order. The diffraction order of the excitation waveguide mode in the resonant waveguide grating structure is also-1 or +1, and evanescent waves are provided at the incident (reflecting) side of the grating and conduction waves are provided at the transmission side. When the resonant waveguide mode is coupled into the environment at the transmission side of the grating, the resonant waveguide mode does not need to be diffracted again and interferes with the directly transmitted-1 order (or +1 order) to couple the energy of the resonant mode into the high diffraction order, namely the-1 order (or +1 order), and high diffraction efficiency is obtained; the coupled-in zero-order diffraction is diffracted again by the grating, so the relative coupling efficiency is low. In the resonant grating waveguide structure, the high-refractive-index layer is equivalent to a multi-beam reflection and transmission cavity, and the thickness of the high-refractive-index layer is adjusted, so that destructive and constructive interference can respectively occur when diffraction of 0 order and diffraction of-1 order (or +1 order) pass through the high-refractive-index layer, the resonant diffraction efficiency of-1 order (or +1 order) is higher, and the diffraction efficiency of 0 order and diffraction of +1 order (or-1 order) is lower. The resonant grating waveguide structure of the invention couples the resonant waveguide energy into high diffraction order through the two processes to obtain high diffraction efficiency high order resonant diffraction. As shown in fig. 5, the-1 st order (or +1 st order) diffraction efficiency exceeds 60%, high-order resonance diffraction is realized, and the light efficiency of the waveguide display device is improved; the low refractive index layer 24, coupled into the resonant waveguide grating 120, serves to reduce the "leakage" of resonant waveguide mode energy, thereby reducing the full width at half maximum of the resonant line, while reducing background stray light.
In order to realize color display, the present invention can adjust various parameters of the coupled resonant waveguide grating 120 to make the red, green and blue lights generate resonant diffraction under the same diffraction angle, thereby eliminating color nonuniformity caused by dispersion effect. For example, the incoupling resonant waveguide grating 120 designed for green light at a wavelength of 520nm, is designed to obtain-1 order resonant diffraction at an incident angle of 11 °, with a full width at half maximum of the resonance line of about 8.5 °, as shown in fig. 5. On the basis, the grating period and the height of the grating layer are changed and the ratio of the wavelength lambda of the incident light to the grating period a is kept the same, namely lambdaR/aR＝λG/aG＝λB/aBThe red light (wavelength of 630nm) and the blue light (wavelength of 450nm) are also subjected to resonance diffraction at the same incident angle, and the diffraction angles are also the same as known from the grating formula. Meanwhile, the thicknesses of the low refractive index layer and the high refractive index layer are adjusted, so that the resonance diffraction efficiencies of the red light, the green light and the blue light are nearly the same, the rainbow effect is eliminated fundamentally, and high-color-purity full-color display is realized, as shown in fig. 7. All structural parameters are calculated by RCWA simulation to give optimized results. Fig. 8 shows the-1 order diffraction efficiency as a function of wavelength for an incoupling resonant waveguide grating designed for red, green and blue light at the same angle of incidence (11 °).
In order to realize a large field angle FOV in AR display application, the resonant waveguide grating has the advantage that the resonant diffraction angle of the resonant waveguide grating is continuously changed within a certain range by adjusting parameters such as the grating period of the grating layer, the height of the grating layer, the thicknesses of the high refractive index layer and the low refractive index layer and the like. Still taking the incoupling resonant waveguide grating corresponding to 520nm green light as an example, the resonant diffraction angle is changed by adjusting the grating period of the grating layer and the thickness of the low refractive index layer, as shown in fig. 9, so that the resonant diffraction angle is continuously changed from-17 ° to +17 °, and the thickness of the high refractive index layer is adjusted so that the diffraction efficiencies of different resonant diffraction angles are nearly the same. Thus, the large field angle FOV is realized by splicing several similar gratings.
Typically, the angular dispersion of resonant diffraction corresponds to the wavelength dispersion. Taking the above-mentioned incoupling resonant waveguide grating for green light with a wavelength of 520nm as an example, in fig. 5, the full width at half maximum of-1-order resonant diffraction is about 8.5 °, and in order to obtain the chromatic dispersion of the incoupling resonant waveguide grating for the wavelength, the change of the grating diffraction efficiency with the wavelength is studied at a fixed incident angle for the incoupling resonant waveguide grating with the same structure. FIG. 10 shows RCWA simulation results of-1 order resonant diffraction varying with wavelength in the range of 400-700 nm at different incident angles. It can be seen from fig. 10 that the full width at half maximum of the resonance diffraction spectrum corresponding to a certain incident angle is between 40 nm and 50nm, and when the incident angle changes from 7 ° to 15 ° (corresponding to the angle range of the full width at half maximum of the diffraction efficiency varying with the angle in fig. 5), the variation range of the center wavelength of the resonance diffraction is 500 nm to 533nm.
In particular, the refractive index n of the transparent waveguide substrate in the resonant grating waveguide structure of the present inventionsThe material can be between 1.5 and 2.0, but is not limited to the range, and the selected material is a low-loss light-transmitting material, but is not limited to a certain material; also for the refractive index n of the grating layerGThe material can be between 1.5 and 2.0, but is not limited to the range, and the selected material is a low-loss light-transmitting material, but is not limited to a certain material; refractive index n of low refractive index layerLCan be between 1.0 and 1.5, but is not limited toThis range; refractive index n of high refractive index layerHCan be between 2.0 and 4.0, but is not limited to this range; refractive index n of transparent waveguide substratesRefractive index n of grating layerGAnd refractive index n of high and low refractive index layersH、nLThe relationship between them is: n isL<ns(nG)<nHWherein n iss、nGThe values may be the same or different. The resonant grating waveguide structure can be applied by selecting different materials of high and low refractive index layers, grating layer and transparent waveguide substrate according to actual conditions, for example, the high refractive index layer can be TiO but not limited to2、Ta2O5、Si、ZnS、ZrO2、Al2O3And the like. In the RCWA simulation calculations shown in fig. 5 to 12, the refractive index of the transparent waveguide substrate is 1.9, the refractive index of the grating layer is 1.9, the low refractive index layer is LiF (refractive index 1.39), and the high refractive index layer is TiO2The material has good dispersibility, and the refractive indexes of the material corresponding to red, green and blue light are 2.5-2.8. Aiming at red, green and blue light, the grating period of the grating layer is 300-600 nm, the height of the grating layer is 40-200 nm, the thickness of the high refractive index layer is 30-300 nm, and the thickness of the low refractive index layer is 30-200 nm. The efficiency of-1 (or + 1) order resonance diffraction using the above parameters is about 60%. It should be noted that the "energy leakage" of the resonant waveguide mode can be controlled by changing the grating parameters and the grating material, and the material of the high and low refractive index layers, as required, to obtain higher resonant diffraction efficiency. For example, the efficiency of resonance diffraction can be brought to nearly 100% by adjusting the refractive index and thickness of the low refractive index layer and the height of the grating layer, and the width of the resonance line can be adjusted as required.
It should be understood that the above description mostly uses the incoupling resonant waveguide grating 120 as an example, and the structure and various parameters of the outcoupling resonant waveguide grating 130 are the same as those of the incoupling resonant waveguide grating 120, and the outcoupling resonant waveguide grating 130 may be set according to the determined structure and parameters of the incoupling resonant waveguide grating 120.
In summary, the resonant grating waveguide structure of the embodiment of the present invention has at least the following advantages:
(1) the sub-wavelength grating structure is combined with the transparent optical waveguide, so that the optical waveguide is applied to near-eye display devices such as AR and VR. The integral structure includes a transparent waveguide substrate for conducting light, an incoupling grating and an outcoupling grating. The multilayer resonant waveguide grating is integrated with the transparent waveguide substrate, and the transparent waveguide substrate is the substrate for coupling-in and coupling-out gratings at the same time.
(2) The grating period is sub-wavelength, i.e. the grating period is less than the wavelength in vacuum. The presence of a finite order of diffraction of the transmission allowed in the "resonant grating waveguide structure" is different from other optical diffraction elements, since the grating period is smaller than the wavelength of the incident light. The resonant waveguide grating is different from other diffraction type gratings or optical diffraction elements, the grating parameters and other structural parameters meet the resonance condition through strict calculation, namely, the resonant diffraction is generated only under a certain wavelength and an incident angle, and the diffraction efficiency of light with other wavelengths and incident angles is close to zero. When incident light is diffracted by the grating, wave vectors are matched with intrinsic modes of the grating structure, resonance occurs, and a resonance waveguide mode is excited in the grating structure. Due to the periodicity of the grating, the resonant waveguide mode gradually "leaks" energy during propagation and couples into the propagating diffraction mode of the grating by diffraction of the grating to produce resonant diffraction. The "resonant grating waveguide structure" is highly transparent to ambient light, since there is little diffraction for light of other wavelengths and angles of incidence. This feature is particularly important for AR display applications, where high penetration allows AR glasses to give users high quality digital images without affecting the view of real scenes.
(3) Through the design of the grating material refractive index, the grating period, the grating height, the duty ratio of the grating layer and the refractive index and the thickness of the low refractive index layer, strict RCWA calculation is carried out, so that the coupled-in resonant waveguide grating 120 and the coupled-out resonant waveguide grating 130 can both generate a resonant waveguide mode, then a high refractive index layer is added, through the design of the refractive index and the thickness of the high refractive index layer, through further RCWA optimization calculation, 0-level and high diffraction level, such as-1 level (or +1 level), through respectively destructive interference and constructive interference in the high refractive index layer with a certain thickness, the resonant diffraction is further optimized, most of the energy of the resonant waveguide mode is coupled into the high diffraction level of-1 level (or +1 level), and high-level resonant diffraction is generated, and the requirement of AR display on the total reflection angle is met.
(4) The resonant grating waveguide structure of the present invention can make the red, green and blue light undergo resonant diffraction under the same diffraction angle by adjusting various structural parameters of the coupled-in resonant waveguide grating 120 and the coupled-out resonant waveguide grating 130: adjusting the grating period of the grating layer and keeping the ratio of the wavelength of the incident light λ to the grating period a the same, i.e. λR/aR＝λG/aG＝λB/aBThe diffraction angle is the same at the moment, and the thicknesses of the low refractive index layer and the high refractive index layer are adjusted simultaneously, so that the resonance diffraction efficiency of the red light, the green light and the blue light is close to the same, the rainbow effect is eliminated fundamentally, and the high-color-purity full-color display is realized.
(5) In the resonant grating waveguide structure, the high-refractive-index layer with the set thickness is calculated to enable transmitted-1 st order (or +1 st order) diffraction to have higher diffraction efficiency, and enable 0 st order and +1 st order (or-1 st order) diffraction to have lower diffraction efficiency, so that the energy of a resonant waveguide mode is coupled into-1 st order (or +1 st order) diffraction.
(6) The resonance grating waveguide of the present invention can change the resonance diffraction angles of the incoupling resonance waveguide grating 120 and the outcoupling resonance waveguide grating 130 by adjusting the structural parameters, and meet the requirement of large field angle FOV in AR display applications: the resonance diffraction angle is changed by adjusting the grating period of the grating layer and the thickness of the low refractive index layer (as shown in fig. 11), so that the resonance diffraction angle is continuously changed within a certain incident angle range, and the diffraction efficiency of different resonance diffraction angles is approximately the same by adjusting the thickness of the high refractive index layer. Thus, the large field angle FOV can be realized by splicing several similar gratings in structure. As shown in fig. 9, the resonant diffraction angle of the resonant grating waveguide varies continuously from-17 ° to +17 °, with a FOV greater than 34. When in actual application, canThe resonant diffraction angle is adjusted over a larger angular range as needed to obtain a larger FOV. FIG. 12 shows the wavelength at 520nm, the thickness of LiF is 70nm, the grating period is 400nm, the duty cycle is 50%, and the resonance diffraction angle is determined by the high refractive index layer (TiO) when the grating height is 100nm2) The change in thickness. For example, an incident wavelength of 520nm, if chosen to produce resonant diffraction at 11 °: the grating period 400nm and the LiF thickness 70nm were chosen as in FIG. 11, and the phase-matched high-index layer (TiO) was chosen as in FIG. 122) The thickness of (a) may be 50nm, 160nm or 270nm, so that the highest resonance diffraction efficiency is obtained.
(7) The resonant waveguide grating structure can adjust the diffraction efficiency of resonant diffraction and the spectral line width of a resonant spectrum by changing the structural parameters, grating materials and high and low refractive index layer materials according to actual needs, and the highest 1-level (-1-level) diffraction efficiency can reach 100%.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims. The information disclosed in this background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.
Claims (9)
1. A resonant grating waveguide structure, comprising:
at least one waveguide structure layer, each waveguide structure layer is provided with at least one pair of an in-coupling resonant waveguide grating (120) and an out-coupling resonant waveguide grating (130); wherein,
the coupling-in resonant waveguide grating (120) and the coupling-out resonant waveguide grating (130) are both multilayer structures and are respectively composed of a grating layer, a low refractive index layer, a high refractive index layer and a transparent waveguide substrate (110) which are sequentially stacked from top to bottom: the grating period of the grating layer is in a sub-wavelength range lower than the wavelength of incident light in vacuum;
the transparent waveguide substrate (110) of the coupling-in resonant waveguide grating (120) and the coupling-out resonant waveguide grating (130) is the same transparent waveguide substrate which is shared;
in the incoupling resonant waveguide grating (120) and the outcoupling resonant waveguide grating (130), the grating period, the height and the duty cycle of the grating layer and the thicknesses of the high refractive index layer and the low refractive index layer satisfy the requirement that the resonant waveguide grating generates a resonant mode for light with corresponding wavelength and incident angle;
the thicknesses of the high refractive index layer and the low refractive index layer meet the phase matching condition, so that the energy of a resonance mode can be coupled into a high diffraction order and the highest diffraction efficiency can be achieved.
2. A resonant grating waveguide structure according to claim 1, wherein the waveguide structure layers are three layers, the three layers being stacked from top to bottom, each layer diffractively propagating light of a different color.
3. The resonant grating waveguide structure of claim 1, wherein the in-coupling resonant waveguide grating (120) and the out-coupling resonant waveguide grating (130),
the refractive index of the grating layer is smaller than that of the high refractive index layer and larger than that of the low refractive index layer;
the transparent waveguide substrate (110) has a refractive index less than that of the high refractive index layer and greater than that of the low refractive index layer.
4. A resonant grating waveguide structure according to claim 3, wherein the refractive index of the grating layer is the same or different from the refractive index of the transparent waveguide substrate (110).
5. A resonant grating waveguide structure according to claim 3 or 4,
the refractive index of the high refractive index layer is 2.0-4.0;
the refractive index of the low refractive index layer is 1.0-1.5;
the refractive index of the grating layer is 1.5-2.0
The refractive index of the transparent waveguide substrate (110) is 1.5-2.0.
6. A resonant grating waveguide structure according to any one of claims 1 to 4,
the height of the grating layer is 40-200 nanometers;
the thickness of the low refractive index layer is 30-200 nanometers;
the thickness of the high refractive index layer is 30-300 nanometers.
7. A resonant grating waveguide structure according to any one of claims 2 to 4, characterized in that the ratio of the incident light wavelength λ of the in-coupling (120) and out-coupling (130) resonant waveguide gratings of the waveguide structure layers arranged for different colors of light to the grating period a of the grating layers satisfies λR÷aR= λG÷aG= λB÷aBSo that the diffraction angles are the same at the same incident angle; wherein λ isRIncident light wavelength of incident red light, aRA grating period set for red light; lambda [ alpha ]GIs the incident light wavelength of the incident green light, aGA grating period set for green light; lambda [ alpha ]BIncident light wavelength of incident blue light, aBThe grating period set for blue light.
8. A resonant grating waveguide structure according to any one of claims 1 to 4, wherein the grating layer is clad with a low refractive index light transmissive outer layer or is directly exposed to air or vacuum.
9. A near-eye display device characterized in that the resonant grating waveguide structure of any one of claims 1 to 8 is employed as a grating waveguide device.
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| CN114578561B (en) * | 2022-01-27 | 2024-03-26 | 东南大学 | Large-view-field high-brightness holographic waveguide system based on multilayer body grating and preparation method thereof |
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| CN115308827A (en) * | 2022-07-25 | 2022-11-08 | 南阳利达光电有限公司 | Diffraction grating structure with diffraction efficiency at each level capable of being freely regulated and controlled |
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